1. Field of the Invention
The invention relates to torque converters and more particularly to a bridge coupling in a hydrodynamic torque converter.
2. Description of the Related Art
DE 44 23 640 A1 discloses a bridge coupling on a hydrodynamic torque converter that comprises a piston arranged in an axially movable fashion on a housing hub that serves as a radial bearing. The piston can be moved in the usual manner between one operating position, wherein the piston is actively connected via friction linings to the support surface of a drive-side radial flange of the converter housing, and a second operating position, wherein the piston releases the friction linings. The piston is connected to a star wheel attached to the housing hub via axially elastic elements, which consist of a plurality of tangential leaf springs secured along the circumference. The tangential leaf springs allow the aforementioned axial movement of the piston, but prevent a rotary movement of the piston relative to the star wheel. The springs can be prestressed so as to either stress the piston in the direction of the drive-side radial flange of the converter housing or draw the piston toward a turbine wheel of the torque converter.
The aforementioned star wheel, which holds the tangential leaf springs, is usually calked to the housing hub and has a tooth system, as does the housing hub itself, to protect against rotation relative thereto. As a result, both the housing hub and the star wheel are relatively complicated.
The tangential leaf springs are connected at one end to the star wheel and at the other end to the piston. These connections are established by riveting. The two connection points, on the one hand, and the tangential leaf springs, on the other, result in the addition of tolerances, so that a broad tolerance band is created. It is also problematic that a radial offset can result from the riveting of the tangential leaf springs to the piston and the star wheel. As a result, on one side of the housing hub, the piston compresses a seal in the housing hub (FIG. 1) such that steel runs on steel, while, on the opposite radial side, a clearance remains that permits the passage of hydraulic fluid.
The prestressing of the tangential leaf springs creates a further problem: In modern vehicles with markedly low idle speeds, the hydropump that supplies the converter circuit with hydraulic fluid, cannot build up sufficient pressure to overcome the force of the tangential leaf springs and to elevate the piston completely from the friction linings upon detachment of the bridge coupling. As a result, the engine can stall. This problem occurs particularly in the case of elbowed tangential leaf springs. In addition, such leaf springs have the disadvantage, due to their axial connection between the piston and the carrier star, of requiring a large axial structural space. Moreover, in order to advantageously accommodate the tangential leaf springs, both the piston and the carrier star must be embodied flatly in the radial extension regions that hold the springs, as a result of which the piston loses rigidity compared with a convex embodiment.